1. Field of the Invention
The invention relates to a worm-rack type transmission device which has a worm wheel and a linear rack to transmit the former rotational movement to the latter to convert it as a linear movement, and particularly to a worm-rack type transmission device in which the worm wheel is formed into a frustocone-shaped configuration.
2. Description of Prior Art
In a worm-rack type transmission device well-suited to transfer a large-scaled machine tool as shown in FIG. 7, a worm wheel 50 is placed on a half-nut type rack 51 with a worm tooth 50a engaged against a rack tooth 51a of a linear rack 51. The worm wheel 50 directs its shaft 50b in parallel with an axial direction of the linear rack 51.
Upon rotating the worm wheel 50 by way of an electric motor (not shown), the linear rack 51 moves in its lengthwise direction N to actuate a robot arm (not shown) of the machine tool so as to transfer component parts of machinery.
Since the worm tooth 50a meshes with the rack tooth 51a substantially through a semi-circular area, it is well-suited to transfer elongate machine tools due to an increased transmission power from the worm wheel 50 to the linear rack 51.
However, the structure is such that the worm wheel 50 is placed in parallel with the linear rack 51 so as to restrict a vacant space between the worm wheel 50 and the linear rack 51 for connecting an electric motor and a power transmission mechanism (e.g., gear train) to the worm wheel 50, thus structurally complicating the power transmission mechanism.
As another example of the worm-rack type transmission device, represented is a scroll chuck device 52 as shown in FIG. 8. The scroll chuck device 52 is employed to chuck a work piece in an engine lathe, and has a spiral tooth band 52a set as a scroll on an upper surface of a rotatable table 53. The spiral tooth band 52a is profiled along Archimedean curve and have three rack pieces 54 to be engaged with the tooth band 52a. 
Upon operating a pinion 55 through a handle 57 with the pinion 55 meshed against a circumferential teeth 56, the spiral tooth band 52a rotates together with the table 53 to convergently travel the rack pieces 54 toward a central portion G along a radial direction L so as to clasp a work piece (not shown).
The spiral tooth band 52a meshes with the rack pieces 54 through a larger area upon reciprocally traveling the rack pieces 54 concurrently. However, it is impossible to travel the rack pieces 54 beyond the central portion G to limit an amount of their travel which the rack pieces 54 can move, so as to make the scroll chuck device 52 unsuitable for an extended distance travel.
In order to mitigate the above disadvantages, a semi-open type side door has been introduced by a reference of Laid-open Japanese Patent Application No. 9-328971. The reference discloses a rack-shaped plate 21 and a scroll-shaped tooth 23 defined on a adjustment gear 22. This enables a rack tooth 25 to move beyond a central portion of the scroll-shaped tooth 23.
However, it is necessary to fully turn the scroll-shaped tooth 23 in order to advance rack-shaped plate 21 by one single pitch along the rack tooth 25. This lengthens a sliding distance of the scroll-shaped tooth 23 against the rack tooth 25, thus necessitating a strengthened drive power with no small friction loss accompanied, making it unsuitable to apply the scroll-shaped tooth 23 to the machine tools in the mechanical industry.
In general, in the worm wheel type transmission device with the worm wheel incorporated into a helical gear, the device has been extensively used because of its increased power transmission and self-locking property with a high precision albeit unsuitable for the usage of the extended distance travel.
In the pinion-rack mechanism, although it is suitable for the usage of the extended distance travel, it has no self-locking property and lacks an increased power transmission because of the limited number of teeth in which the pinion meshes with the rack.
With the above in mind, it has been desirable to introduce a power transmission device in which the advantages of the worm wheel type transmission device are taken in and its drawbacks are compensated, while at the same time, the advantages of the pinion-rack mechanism are taken in and its drawbacks are compensated.
Therefore, it is an object of the invention to provide a worm-rack type transmission device which enables to a self-locking property and strengthening a transmission power to insure a high transmission efficiency with a least friction loss so as effectuate a linear drive for an extended distance travel with a high precision.